Iron oxide manufacture



Patented Feb. 13, 1951 UNITED STATES PATENT OFFICE IRON OXIDEMANUFACTURE No Drawing. Application April 10, 1948, Serial No. 20,357

4 Claims. (01. 23200) The present invention concerns the art of pigmentmanufacture and has special application in the making of iron oxidepigments.

This application is based upon and is a continuation in part of ourcopendin application Serial No. 479,370, filed March 16, 1943, nowabandoned.

Prior processes for making iron oxide pigments, e. g. red iron oxidepigments, have had various defects and, despite modifications from timeto time, have left much to be desired, both from the point of view ofthe efiiciency and workability of the process and also from the point ofview of character and/or quality of the finished products.

Iron oxide pigments have heretofore commonly been made by roasting ironsulfate or iron sulfatecontaining materials in a direct fired rotarykiln.

For many years pigment iron oxide has been manufactured by roastingcopperas, i. e. hydrated ferrous sulfate, FBSO4.7H2O, whereby the sulfuris driven off as S02 and/or SOS leavin behind the iron oxide. Actuallythis calcination is commonly a 2-step process. If ordinary crystalcopperas is fed directly to th kiln in which the roasting is being done,it is inclined to melt and stick to the walls of the kiln before all ofthe water is driven off, thus leading to the phenomenon commonly knownas ringing. Thick layers 0r rings of the ferrous sulfate build up on thewalls of the kiln, become very hard as the moisture is driven off andseriously interfere with the process. For this reason the copperas iscommonly fed first to a kiln which is operated at a temperature too lowto decompose the iron sulfate wherein it gradually loses its moistureand gathers into small lumps or nodules which vary in size from lessthan A, in diameter to more than 1". This anhydrous iron sulfate,commonly known as mother, is then fed to another kiln operated at ahigher temperatur wherein it is decomposed to iron oxide and oxides ofsulfur. Various attempts have been made to accomplish these operationsin a single step, as for example, the process described in PeterFiremans patent, U. S. No. 2,184,738. Even this, however, is chiefly amechanical linking of the two steps that are used in the conventionalprocess since it consists essentially of the operation of two kilns inseries which are fired separately, the first feeding directly to thesecond.

Another difliculty with the conventional process is that the ironsulfate during its dehydration does not become uniform in size butcovers a considerable size range as indicated above and as will bediscussed more fully hereinafter. This lack of uniformity makes itdifficult (or impossible) to control the final calcining operation insuch-a manner that all of the iron sulfate is decomposed without aportion of the iron oxide being over-burned. Ordinarily no attempt hasbeen made in such processes to secure complete decomposition of the ironsulfate, and since even a small portion of iron sulfate would render thepigment valueless for commercial applications, it has been necessary andcustomary to wash it in water, and then to filter and dry the undissolved iron oxide.

Such procedures are expensive and of limited effectiveness.

Suggestions have also been made to manufacture iron oxide pigments fromthe product obtained by treating iron ore with sulfuric acid androasting the product. For example, Ledoux, U. S. Patent No. 360,967,suggests a method for preparing iron oxide pigments by treating pyritessinter, the by-product obtained when iron pyrites is roasted to obtainsulfur dioxide, with sulfuric acid and roasting the mixture of ironsulfate and gangue which is obtained thereby. To our knowledge Ledouxssuggestions were never commercialized prior to the present invention.This was due, we believe, among other things, to the fact that after thepyrites sinter is 50 treated with sulfuric acid and allowed to cool andharden, it is necessary to crush it before it is fed to the kiln. Thisoperation yields a material which is very non-uniform in size andbehaves very poorly in the roasting operation.

When we attempted to commercialize a process of this sort, we found itnecessary to treat the hard rock-like masses which result from the acidtreatment of the ore in a hammer mill or ClllSh-v ing device. Thisresulted in a material which varied in size from more than an inch indiameter to a fine powder. When this was roasted, it was necessary thatall of the iron sulfate be decomposed since the economics of the processdid not allow for the washing and filterin of the roasted material. Thismeant that the roasting had to be carried on for a sufiicient length oftime and at a sufiiciently high temperature completely to decompose allthe iron sulfate even in the center of the larger pieces. This resultedin the over-burning of the smaller pieces so that the product wasseriously lacking in uniformity of color. Other difiiculties also arosewith a ma terial of this sort as a kiln feed. Very fine material would,to substantial extent, be blown from the kiln and thus lost as dust. Insome cases the fine material tended to segregate to the bottom of thecharge as it progressed'through the kiln (e. g. a direct fired rotarykiln) so that it became a more difiicult completely to decompose thanthe intermediate sized pieces since, remaining always at the bottom ofthe charge, it did not have free access to the, heat from the flame. Bycontrol-l ling the crushing or grinding of the impuresul-. fate of ironso as to produce a minimum of fines and later segregating the lumps thatwere too.

large, it was sometimes possible by using a low rate of feed to the kilnto avoid serious overburning of a portion of theiron oxide. and, tosecure substantially complete decomposition of the iron sulfate, but theuniiormlty of, the, prod-- not obtained. still left a great deal to bedesired:

and the crushing operations were so. expensive.

and the rates through the kiln were so lowthat the process was in nosense satisfactory commercially.

While correctly fired iron sulfate is of desired red color, overfiringof some of the, kiln products: makes: them weak and bluish in colorand',.,Qn.- the. other hand, underfiring of other portions ofthe kilnproduct (where this occurs) leaves. them; on the yellow acid side. Thesubstantially complete avoidance of underfired material ac cording toprior methods was virtually certain; to result in an increase in theproportion of, overfiredmaterial in the product, as well as an increasein kiln costs and a decrease in quality which, could not be remedied bywashing.

A salient aspect of our invention provides; for forming into extrudedparticles, short. cylinders or pellets the material which is to becharged, to the kiln or furnace, the extended pellets, or equivalent,being normally controlled; within a predetermined diameter or sizerange.

' It is an object. of our invention to provide a. process of producingpigments by calcination or firin which avoids orgreatly minimizesoverfiring as well as underfiring. A further object. is to provide forstronger color development than: with kiln feeds heretofore employed. Astillfurther object is to greatly increase the kiln orfurnace capacity,especially due to the better and more uniform heat conductivity of theparticles or pellets of material charged into the kiln andthe more:uniform flow of such material result.- ing in better heat distribution.Another object. is; to minimize or obviate stack losses, due to absenceof fines from the charge. A: further; object isto overcome the sticking,building up, or ringing so prevalent in the feed end, or cool: end of;the kiln in prior processes. These. and other, objects and advantageswill be evident. from the description taken as a whole.

We have found that the product obtained-by: treating ore with sulfuricacid can beextrudedv provided certain conditions are rigidly adhered to.If, after the iron ore is reacted with the sulfuricacid and before ithas become cool, for; example at a temperature of the order of, 290-F.-, more or less, it is treated in a, pugmill or: similar device, ityields a plastic mass which; is amenable to the extrusion operation;After;

leaving the pug mill the plastic mass (still: at:

elevated temperature) enters the extruder, which; may be of conventionaldesign, having tWOSC'IeWS' which force the iron sulfate containingmixture against the die plate; which may be of; the multiple orifice type,and extrude it; at a, temperature of the order above mentioned onto; a.conveyor belt. After cooling and drying somet what, these extrusions arefed directly to; thekiln;

By extruding the acid treated ore in this way we have found that, in thefiring or calcining operation, the rate through the kiln can be fullydouble that realized with a feed prepared in any other manner, and thequality of the finished pigment is much better. since over-burning issubstantially avoided or eliminated and undecomposed iron sulfate iscompletely or substantially eliminated.

It will be recognized that the term iron ore as used herein coversnaturally occurring ores such. as. hematite, and also coversiron-shearing materials such as pyrites sinter, which react with:sulfuric acid to form iron sulfate.

The; diameter of the extrusions thus prepared is not extremelycritical,but we have preferred extrusions approximately in diameter. Extrusionssubstantially smaller than this are more difficult. to prepare becauseof the extremely high pressures developed on the die plate and arelikely tobe more friable before and during. the calcination step.Extrusions substantially greater. in diameter are difiicult toroastuniformly to; the: center and result in low rates: through. the kiln.The optimum size will obviously. vary.- somewhat with theexact nature ofthe material; to be fired and the conditions of firing. Hoivever,,ingeneral, the. diameter or small dimension of; the; pellets or extrudedpieceslof. charge will. normally fall: within, or, approximate, therange of; to 4;- inch and usually will,fa1l- Within-.a.

narrower range, say, to inch.

It has been found also that a. plastic. mass. composed largely of ironsulfataand which may be extruded. to form small shaped pieces orextrusions suitable for our calcination process-,may be; prepared bywarming and partially dehydrat-- ing; copperas, The mass is then mixedin; the pug. mill, prior to extrusion.

To obtain extended iron, oxide pigments with; cop eras. as the source ofiron we may also add an. extender to the copperas before it. goestothepug mill: or other plasticizing equipment. and thence: to theextruder. For, example, the. fol-- lowing; mixture may be used Partscopperas -l- 1850. Ammonium sulfate 91. 1d

Hereinabove we havediscussed: advantages gained by. having thematerial,which is. charged; to thekiln furnace or-thelike, of; a reasonable unform, predetermined: size, especially as re,- garcls the;cross-sectional dimensions of oblong pieces. Shape is also ofimportance. inrespect to= heat, transfer in, the-rkiln; and: mechanicalstrength oi'the -partioles or pellets, but; the shapes may v ry quitewidely- Where the piecesor ex;-

tende d,-. pellets or chargev are.- formed by extrusiom they"will-usually be of circular; s uare rectangu lar or triangularcross-section, with the length of the same of less importance andordinarily varying considerably. However the length usually is greaterthan the diameter or cross-sectiona1 dimension and, where the pieces arecircular in cross-section and have a diameter of about inch, they arecommonly 1 to 3 inches in length, though the length can vary outsidethese limits. The smallest dimension of the particles or pellets isadvantageously as nearly uniform as possible. While extrusion is onepractical way of forming the material into sized particles, other Waysof forming sized particles are also contemplated and are illustrated byprocedures and practices well known in the arts of making pills, candy,etc.

While this invention has been illustrated largely in connection with'thepreparation of red iron oxide pigment, it is also applicable in theproduction of other pigments, especially to those where heating andagitation are involved in their production, including titanium oxide,lithopone, zinc sulfide, calcium sulfate, etc. However, the problems ofover-heating and under-heating are more acute in the case of making rediron oxide and other pigments Where a substantial endothermic reactionoccurs during the firing; but the problems are nonetheless present inthe making of various other pigments such as those illustrated above.

While our invention has been illustrated hereinabove in connection withspecific materials, to produce particular pigments, it will beunderstood that it is applicable in the treatment of other materials inthe preparation of similar or different pigments or end products. Inother respects also the specific descriptions given herein are exemplaryand not limitative, and all equivalents and variations, novel over theprior art, are likewise hereby comprehended.

What we claim is:

1. In the art of iron oxide pigment manufacture, the process whichcomprises selecting as a starting material a mass which is composedpreponderantly of iron sulfate and which, upon mixing, is a solidplastic mass at elevated temperatures of the order of 200 F., mixingsaid mass at an elevated temperature to make it uniformly plastic,forming the plastic mass into small, shaped pieces having roundedsurfaces and a substantially uniform small dimension of approximately toinches, allowing the pieces to dry suiiiciently to harden and thencalcining the shaped pieces under conditions (a) wherein all peripheralsurfaces of the shaped pieces are exposed to a substantially uniformheat treatment and (b) wherein the temperature and time of treatment arecontrolled so as to decompose the iron sulfate.

2. In the art of pigment manufacture the process which comprisestreating iron. ore with sulfuric acid so as to convert the same largelyto iron sulfate at an elevated temperature in excess of 200 F., mixingthe resulting iron sulfate containing product at an elevated temperatureto render the same plastic, extruding the plastic product to formextrusions thereof having a diameter of the order of inch and a lengthof the order of 1 to 3 inches, allowing said extrusions to cool and drysufliciently to harden, and then charging said extrusions continuouslyto a rotary kiln wherein the temperature and time of treatment iscontrolled so as to decompose said sulfate.

3. In the art of pigment manufacture the process which comprisestreating iron ore with sulfuric acid at an elevated temperature so as toconvert the ore largely to iron sulfate, mixing the resulting product atan elevated temperature to render it plastic, forming the plasticproduct into small shaped pieces having a substantially uniform smalldimension, allowing the pieces to cool and dry sufficiently to hardenand then charging them continuously to a rotary kiln wherein thetemperature and time of treatment are controlled so as to decompose theiron sulfate.

4. In the art of iron oxide pigment manufacture a process whichcomprises selecting as a starting material a product which, upon mixing,is a solid, plastic mass at elevated temperatures of the order of 200 F.and which is composed preponderantly of iron sulfate, mixing suchproduct at an elevated temperature to make it uniformly plastic, formingthe plastic product into small shaped pieces having rounded surfaces anda substantially uniform small dimension, allowing the pieces to cool anddry sufiiciently to harden and then charging them continuously to arotary kiln wherein the temperature and time of treatment are controlledso as to decompose the iron sulfate.

ROBERT C. JOHNSON. SAMUEL B. BAUMAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 20,655 Monnier June 22, 1858360,967 Lecloux Apr. 12, 1887 513,679 Hemingway Jan. 30, 1894 1,894,368Crowley Jan. 1'7, 1933 1,943,948 Castner Jan. 16, 1934 FOREIGN PATENTSNumber Country Date 404,991 Great Britain Jan. 23, 1934

3. IN THE ART OF PIGMENT MANUFACTURE THE PROCESS WHICH COMPRISESTREATING IRON ORE WITH SULFURIC ACID AT AN ELEVATED TEMPERATURE SO AS TOCONVERT THE ORE LARGELY TO IRON SULFATE, MIXING THE RESULTING PRODUCT ATAN ELEVATED TEMPERATURE TO RENDER IT PLASTIC, FORMING THE PLASTICPRODUCT INTO SMALL SHAPED PIECES HAVING A SUBSTANTIALLY UNIFORM SMALLDIMENSION, ALLOWING THE PIECES TO COOL AND DRY SUFFICIENTLY TO HARDENAND THEN CHARGING THEM CONTINUOUSLY TO A ROTARY KILN WHEREIN THETEMPERATURE AND TIME OF TREATMENT ARE CONTROLLED SO AS TO DECOMPOSE THEIRON SULFATE.